In recent years, ceramic honeycomb structures superior in heat resistance and corrosion resistance have been used as a dust-collecting filter for environmental management (e.g. pollution control), product recovery from high-temperature gas, etc. in various sectors including chemistry, electric power, steel and industrial waste treatment. Ceramic honeycomb structures are suitably used, for example, as a dust-collecting filter for use in a high-temperature, corrosive gas atmosphere such as a diesel particulate filter (DPF) for trapping particulates emitted from a diesel engine.
The honeycomb structure used as the above dust-collecting filter is required to have a constitution low in pressure loss and capable of giving a high trapping efficiency. For the requirements, a honeycomb structure wherein at least some of the cells are plugged, for example, as shown in FIG. 2, a honeycomb structure 21 wherein an inlet end face B and an outlet end face C are plugged alternately by plugged portion 22, is used. In the honeycomb structure 21 having such a structure mentioned above, a gas G1 to be treated is introduced into cells 23 from the inlet end face B, dust and particulates in the gas are trapped by partition walls 24; meanwhile, the gas which has entered adjacent cells 23 through the porous partition walls 24, is discharged as a treated gas G2 from the outlet end face C, as a result, the treated gas G2 wherein dust and particulates in the gas G1 to be treated are removed can be obtained.
A honeycomb structure having above described plugged portions can be obtained by: forming plugged portions by dipping an end face of a honeycomb structure having cells functioning as fluid passages in slurry containing at least ceramic particles and dispersion medium in a container, and pressing the honeycomb structure against an inner bottom surface of the container to force the slurry into at least some of the cells; and bringing out the honeycomb structure having the plugged portions formed therein from the container by, for example, picking up the honeycomb structure directly.
The honeycomb structure produced by the above process, however, has had a problem of yielding defect in the plugged portions. FIG. 3 is schematic enlarged sectional drawings of the vicinity of the inlet end face B of a honeycomb structure 21. FIG. 3(i) shows a plugged portion 22 to be formed satisfactorily. In this plugged portion 22, however, a shrunk dent 26 has generated as shown in FIG. 3(ii) and, in an extreme case, there has appeared a hole 27 passing through the plugged portion 22 as shown in FIG. 3(iii).
When the shrunk dent 26 generates, there is an inconvenience of the reduced reliability of the plugged portion 22; when there appears the hole 27 passing through the plugged portion 22, dust and particles leak through the hole 27, when it is used as a dust-collecting filter, it becomes impotent as a filter. Hence, this problem has heretofore been avoided by, as shown in FIG. 3(iv), forcing a ceramic slurry (for formation of plugged portion 22) excessively into the cell 23 to make larger the depth (d) of the plugged portion. When the depth (d) of the plugged portion is made larger, however, the surface area of the partition walls 24 separating the cells 22 from each other, that is, the area of filtration is reduced, which is not preferred.